The present invention relates to filters for removing contaminants from swimming pools.
Swimming pool sand filters typically include both a tank containing sand and an underdrain positioned at the bottom of the tank. The term xe2x80x9cunderdrainxe2x80x9d denotes the apparatus which, when installed into the sand-filled tank, controls the transmission of the swimming pool water. The underdrain can perform either of two distinct functions depending on the direction of the water flow. If the water initially flows through the sand, and only subsequently into the underdrain, the system operates to filter contaminants from the water. Contaminants in the water are trapped in the sand as they flow towards the underdrain. The filtered water will then enter the underdrain and be returned to the swimming pool.
If the flow is reversed such that water from the pool is initially piped into the underdrain at the bottom of the sand-filled tank, and only then dispersed into the sand, the system operates to wash contaminants from the, sand. This type of operation is known as backwashing.
In order to maximize performance, the underdrain should function to force the water to flow through the largest possible portion of the sand so as to produce the greatest filtering action or, alternatively, so as to produce the greatest cleansing action.
Prior art underdrain designs as illustrated in FIG. 1 typically consist of a central manifold 10 that directs the water to or from multiple lateral projections 20 that extend from the manifold 10 like the spokes on a wheel hub. The lateral projections 20 include small slots or perforations which permit the passage of water, but prevent the passage of sand 30. Accordingly, the water is transferred to or from the sand 30 in a spoked pattern that roughly covers the area of the tank bottom.
Since the distribution pattern resembles radial spokes, the total flow of water at any radial distance from the manifold remains a constant. For example, if the total flow is 10 gal/min at 5xe2x80x3 from the center of the tank 40, then the total flow at 8xe2x80x3 from the center of the tank 40 would also be 10 gal/min. However, since the circumference at 8xe2x80x3 is around 50.3xe2x80x3 and is 60% larger than the 31.4xe2x80x3 circumference at 5xe2x80x3, at the greater radial distance the same total flow is distributed over a significantly larger perimetral area.
This problem is accentuated during the backwashing operation. The high flow of water at the center of the tank 40 may form a tunnel through the sand, which will then draw even more water from the outside regions of the tank 40 to this path of least resistance. FIG. 2 illustrates this possibility. The result leaves sand at the outside edge of the tank 40 virtually unwashed and consequently reduces the filtering capacity of the system. As a result, the user must backwash more often in order to maintain the filtering capacity at an acceptable level.
The spoked design also makes installation of the underdrain somewhat difficult. Since the spoked underdrain cannot fit through the tank opening in one piece, the laterals 20 usually have to be screwed into the manifold 10 after they are placed inside the tank 40. This operation must typically be performed using only one hand that is inserted through a relatively narrow opening at the top of the tank 40.
One prior art design utilizes hinged or pivoted connections between the lateral projections and the manifold, such that the complete assembly can be installed at one time. The installer then only has to reach into the tank to pivot the laterals downward into position. The problem with this design is that once installed, the hinges eventually become clogged with sand and lose functionality. Once this happens, the underdrain cannot be removed without damaging it.
It is an object of the present invention to provide an underdrain that both can be installed and/or removed in one piece, without any disassembly or hinge manipulation, and that distributes the water flow to the entire tank bottom area. The one-piece underdrain design according to the present invention addresses both performance and installation issues. Rather than lateral projections, the design primarily relies on a peripheral ring to transfer the water to the sand. The ends of the ring are not joined so that the ring is not closed. The ring is also bent into a helical shape which allows the ring to be screwed into an opening with a smaller diameter than that of the ring. As a result no assembly by the user is required in order to complete installation.